This introduces a bit array to keep track of the slices of a FEC group
that have been received so far. This allows to quickly look up whether
the current slice is already present in the FEC group, in which case we
simply ignore it.

This avoids errors of the kind

unregister_task: unregistering fecdec (slot 0) (invalid index vector)

which have been observed with the UDP transport.

The patch also cleans up add_slice() a bit: It replaces the calls to para_malloc()
and memset() by a simple call to para_calloc() and initializes slice_num earlier
so that it can be used throughout the function.